Visualization of Confined Electrons at Grain Boundaries in a Monolayer Charge-Density-Wave Metal.

1D grain boundaries in transition metal dichalcogenides (TMDs) are ideal for investigating the collective electron behavior in confined systems. However, clear identification of atomic structures at the grain boundaries, as well as precise characterization of the electronic ground states, have largely been elusive. Here, direct evidence for the confined electronic states and the charge density modulations at mirror twin boundaries (MTBs) of monolayer NbSe, a representative charge-density-wave (CDW) metal, is provided.